Multiple algorithms are available, SIMPLE* and COMPLEX*. SIMPLE is a very fast algorithm and can return queries in <10ms on a 10m wikipedia index (complete index).
It can also scale to 2 billion docs. COMPLEX is a more complex algorithm so is a little
slower compared to the SIMPLE, but can also still return queries in < 10ms on a 10m wikipedia index (complete index).
COMPLEX is more accurate and should be able to give you the best rankings as compared to SIMPLE. SIMPLE is also very good and may be well suited than COMPLEX for some type of queries.

In document mode, it ranks documents such as HTML, Wikipedia, Word/PDF docs relevantly while in Product mode, a term's occurrence is taken into account and scored accordingly. So titles starting with "wii console" are ranked first, and the others rank lower as the occurrence of "wii console" shifts in the title or gets reversed, see below:

Wii Console and Wii Fit Plus with Balance Board Bundle (Nintendo Wii)

Wii Console System with Wii Sports Resort Game with TWO MotionPlus Attachments

The RankingAlgorithm has been integrated into elasticsearch so that either Lucene or the RankingAlgorithm can be used to do the search. The RankingAlgorithm scoring or working does not break any of the existing functionality, so shrad, faceting, highlighting, etc. still work as before. RankingAlogirhtm only uses Lucene Apis to retrieve the terms from the index and uses its own ranking and scoring to rank the documents. The scoring is very friendly and easy to follow.

Try Autocomplete using Solr with RankingAlgorithm, similar to Google/Yahoo/Bing's autocomplete (It is free), Give it a try

Installing and Using elasticsearch with RankingAlgorithm

Install elasticsearch as before, no changes to the existing installation steps (see elasticsearch docs
for installation). No changes to the way you query or use elasticsearch. The change is when you
initiate a query, the search uses RankingAlgorithm instead of Lucene. You can still use
Lucene by adding "-Dlibrary=lucene" to use Lucene as before. You can download elasticsearch with RA from here and follow the steps as in the download docs either on the elasticsearch website or from here.

ver 1.4.8, works with Lucene 4.3 [md5sum:a40e448358657de69291ee1c517d3a64], 2013/06/02 (1.4.8.20130602.1)
(Has built in components Edge/Infix autocomplete with source. Support for glob/regular expressions in boolean queries)

4. Add lucene-core-4.x.0-ALPHA.jar and rankingalgorithm40_1.4.4.jar to the classpath
5. Run your previous Lucene examples, applications with no changes but using the RankingAlgorithm for searches instead of Lucene library.